FIG. 29 is a partly sectional perspective view of conventional electrolytic capacitor 5001 disclosed in Patent Literature 1. Capacitor element 11 includes electrode foil 12 as an anode, electrode foil 13 as a cathode, and separator 14 provided between electrode foils 12 and 13. Electrode foils 12 and 13 and separator 14 are rolled together Electrode foil 12 includes an aluminum foil having a surface having an effective area enlarged by etching, and a dielectric oxide film formed on the surface of the aluminum foil by anodizing. Electrode foil 13 includes an aluminum foil having a surface having an effective area enlarged by etching. Capacitor element 11 is impregnated with electrolyte 17. Capacitor element 11 and electrolyte 17 are accommodated in case 18 made of aluminum. Case 18 is sealed with sealing plug 19. Lead wires 15 and 16 are connected to foils 12 and 13, respectively, and exposed to the outside of case 18.
In conventional electrolytic capacitor 5001, in order to obtain a large capacitance, the surface of the electrode foil is roughened by etching so as to increase the effective surface area. However, the surface area cannot increase beyond the technical and physical limitations in etching technique and mechanical strength of the electrode foils, hence preventing the capacitance from increasing.
The surface of aluminum electrode foil 12 is roughened by etching or vapor deposition.
Patent Literature 2 discloses an etching method. In this method, an aluminum foil undergoes electrolytic etching in an etching solution containing chlorine ions. First, the aluminum foil undergoes direct-current (DC) electrolytic etching process and then a dipping process. After that, the aluminum foil undergoes alternating-current (AC) electrolytic etching process with gradually increasing current density, and then, with a constant current density. This method allows the aluminum foil to have predetermined initial pits before the start of the AC electrolytic etching regardless of characteristics change of the foil. This provides electrode foil with high quality stably. However, the surface area cannot increase beyond the technical and physical limitations in processing accuracy in etching and mechanical strength of the electrode foil. That is, further increase in capacitance cannot be expected.
Patent literature 3 discloses a vapor deposition method. In this method, a base undergoes vapor deposition in a processing chamber. The chamber has an ambient gas containing oxygen or oxide in which aluminum is evaporated, then condensed and solidified. The pressure on the deposition zone in the chamber is within a range from 2.8 Pa to 0.3 Pa. In the ambient gas in the chamber, a vapor-deposited film is formed at a rate of thickness increase ranging from 0.03 μm/sec. to 0.2 μm/sec. on one or both surfaces of the base, thereby providing the aluminum foil having a rough surface. However, the vapor-deposited film is made of particles with weak bonding, by which a neck section connecting the particles can be broken during the anodizing on the aluminum foil. If the neck section is broken, a current flow is blocked at the broken position and therefore a predetermined capacitance cannot be obtained. Further, a mechanical stress exerted on the electrode foil in the rolling process can damage the vapor-deposited layer of the foil, thus preventing the capacitor element from being produced by a rolling process.
Recently, as increase in electronic devices operating at high frequencies, there is a growing demand for manufacturing a capacitor having a small impedance even in a high-frequency range. To meet the needs, a solid electrolytic capacitor in which conducting polymer with high electrical conductivity is used for solid electrolyte has been developed. Manufacturers seek to produce improved solid electrolytic capacitors having small equivalent series resistance (ESR) and small equivalent series inductance (ESL).
FIG. 30 is a perspective view of conventional solid electrolytic capacitor 5002 disclosed in Patent Literature 4. FIG. 31 is a plan view of capacitor element 411 of solid electrolytic capacitor 5002. Capacitor element 411 has a flat-plate shape. The surface of electrode foil made of aluminum as valve metal is roughened to obtain anode foil. A dielectric oxide film is formed on the surface of the anode foil. After that, insulating resist 412 is disposed on the surface of the anode foil to separate the anode foil into anode section 413 and a cathode forming section. Solid electrolyte of conductive polymers is formed on the dielectric oxide film in the cathode forming section. A carbon layer is formed on the solid electrolyte. A silver paste layer is formed on the carbon layer. The carbon layer and the silver paste layer constitute a cathode layer, providing cathode section 414. Anode section 413, cathode section 414, and resist 412 disposed between a node section 413, cathode section 414 are arranged in a longitudinal direction of the anode foil.
Anode common terminal 415 is connected to anode section 413 of capacitor element 411. In solid electrolytic capacitor 5002, plural capacitor elements 411 are stacked on anode common terminal 415. Anode section 413 of each capacitor element 411 is connected to anode common terminal 415 by, e.g. laser welding.
Cathode common terminal 416 is connected to cathode section 414 of capacitor element 411. Cathode common terminal 416 has bent section 416A at which both sides of terminal 416 are bent upward. Conductive adhesive 417 electrically connects between cathode common terminal 416 and cathode section 414 of each capacitor element 411, and connects between cathode sections 414 of capacitor elements 411.
Insulating package resin 418 covers multi-layered capacitor elements 411 such that anode common terminal 415 and cathode common terminal 416 are partly exposed to the outside. The exposed portions of anode common terminal 415 and cathode common terminal 416 are bent along the side to bottom of package resin 418 so as to provide anode terminal 415B and cathode terminal 416B on the bottom, thus providing surface-mount solid electrolytic capacitor 5002.
In solid electrolytic capacitor 5002, as described above, conductive adhesive 417 connects between bend 416A of cathode common terminal 416 and between cathode section 414 of capacitor element 411. This structure decreases an internal resistance of an entire structure of multi-layered capacitor elements 411, decreasing the ESR.
In solid electrolytic capacitor 5002, however, like in electrolytic capacitor 5001, electrode foil undergoes etching to have roughness on its surface and increases an effective surface area. The surface area cannot increase beyond the technical and physical limitations in etching technique and mechanical strength of the electrode foil. That is, further increase in capacitance cannot be expected.
In response to the demand for increase in capacitance, a rolled solid electrolytic capacitor has been introduced on the market. Compared to the multi-layered structure, the rolled structure is easy to increase capacitance. Patent Literature 5 discloses a conventional rolled solid electrolytic capacitor. This capacitor includes anode foil and cathode foil rolled on one another via a separator.
The separator, as is used in the conventional electrolytic capacitor employing electrolyte solution for electrolyte, is formed of the following plug: “electrolytic” paper formed of Manila fiber or kraft paper; glass fiber non-woven cloth; and insulating sheets, such as non-woven cloth composed mostly of a resin formed by a dry melt-blowing method. Instead of the plugs above, the separator may be synthetic resin, such as non-woven cloth made of polyvinyl-alcohol-based resin or non-woven cloth mixed the polyvinyl-alcohol-based resin with other resin.
In solid electrolytic capacitor 5002, however, like in electrolytic capacitor 5001, electrode foil undergoes etching to have roughness on its surface and increases an effective surface area. The surface area cannot increase beyond the technical and physical limitations in etching technique and mechanical strength of the electrode foil. That is, further increase in capacitance cannot be expected.
Patent Literature 1: JP2005-223122A
Patent Literature 2: JP09-171945A
Patent Literature 3: JP07-15864B
Patent Literature 4: JP2003-45733A
Patent Literature 5: JP10-340829A